The present invention relates generally to current mirrors and more specifically to an improved high voltage current mirror.
Current mirrors generally include an input path including an input transistor having a transdiode connection between its collector and base and an output transistor whose base is in common with the base of the input transistor and whose emitter is common with emitter of the input transistor and connected to a reference voltage. The output current is taken from the output transistor's collector. The operation of the current mirror is to control the output current to match the input current. A typical example is illustrated in FIG. 1 wherein the input transistor is Q1 receiving the input current I.sub.i from the input terminal to the collector and transdiode connection and an output transistor Q2 having output collector current I.sub.O connected to the output terminal to which is also connected to a load Z.sub.L which is connected to a voltage V. The collector to emitter voltage V.sub.CE of transistor Q1 is typically 0.7 volts. The collector to emitter voltage V.sub.CE of output transistor Q2 is equal to V-I.sub.O Z.sub.L. For small voltage differences between V and V.sub.REF, the current mirror operates to maintain the input and outputs currents equal. When the voltage difference becomes substantial, the output current I.sub.O becomes larger due to Early voltage effects and therefore the current out does not equal the current in and the V.sub.CE of Q2 becomes larger compared to V.sub.CE of Q1.
Thus there exists a need for a current mirror which is capable of operating at high voltage differentials.